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Re: PLEUROCOELOUS VERTEBRAE IN SAUROPODS



On Sun, 10 Nov 1996 GSP1954@aol.com wrote:

> Sauropod air-sacs where not for cooling, and probably not for structural
> lightning either since giant mammals lack them.

        I don't know that much about cooling, but I find it little short
of impossible to believe that they played no role in structural mechanics!
Giant mammals do not have pleurocoels, but some do have a lot of hollow
space in the bone- if you look at the bone in an elephant's skull, for
example, much of it is made up of thin sheets forming a honeycomb-like
cells. 

> They were probably part of a
> bird-like respiratory system. The main function was to draw air completely
> through the lungs, overcoming the dead air space created by the super long
> trachea.

        So why put them in the bones? Modern birds put a lot of their air
sacs in the abdominal cavity. If hollowing bones served no structural
purpose at all, shouldn't dinosaurs have just had air-sacs in the chest?
Also, T. rex had pleurocoels, and yet it had a very short neck
proportionately. And finally, if some pleurocoels were not in fact open-
and there has been doubt in a few cases- they certainly couldn't have
worked in this manner. 
        I was trying to round up the ref on the closed pleurocoels,
but couldn't. Instead, I found this curious quote which could be applied
to the stability of rearing sauropods:
        
        "Another very interesting feature, not absolutely
unique to sauropods but unusual, is the evolution in the last half of the
back of a bony projection on the center of the rear face of the arch that
fits neatly into a groove on the front of the arch of the succeeding
vertebra (hyposphene-hypantrum articulation). This virtually locks the
vertebrae together and provides great stability". Just ran across that
looking for the pleuocoel ref.in McIntosh's segment in "The Age of
Dinosaurs: Short courses in Paleontology".
         Another interesting feature is that although Diplodocus does
have tail pleurocoels, they are on the VENTRAL side of the centrum! This
appears to mean that they were were experiencing less compression
ventrally than dorsally. Think about what that means. The only way I can
figure out how it must have happened would be if it were in fact leaning
back on the tail as a prop, which would tend to put compression on the top
, tension on the bottom- and *that* just might help explain why there are
those big chevrons, which as I believe Greg has noticed previously, should
be bearing tensional stresses.

        
            =o
             |
             |
              \
               \
                \    
                  +
            \   ///+
             \_o////+
                  ///+
                     O+ <--Top under compression
                     | +
                     |  ++   
                     |    ++
                    ^^      + + ======------------
                        
                        ^
                        |
                         bottom under tension

        The thing is, the tail curves "upward" as the diplodocus leans on
it, and the top side is compressed, and the bottom side is pulled. So this
might explain why there is ventral excavation, if diplodocus did this.

        And I have a possible answer to one of Brian's questions. From
McIntosh's segment in Dinosauria, on the subject of forked neural spines:
"This unusual development apparently arose independently in the two
families and provided a channel for a heavy ropelike muscle, which
connected the skull with the spines of the cervical and dorsal vertebrae
and was useful in raising the skull and neck. Parts of the calcified
muscle are actually preserved in one specimen of Camarasaurus."
        We could still probably argue whether it was a muscle or a
ligament, but it sure sounds like _something_ was running through that
fork! 
        Yet again, if this muscle could help raise the parts of the spine
that it passed through, it is interesting to note that in Camarasaurus, it
is bifurcate to the shoulders, but in diplodocids, it is birfurcate all
the way back to the hips. 
        
        Nick L.